U.S. patent number 4,250,132 [Application Number 06/044,076] was granted by the patent office on 1981-02-10 for material and apparatus for extruding a crosslinked polyolefin product.
This patent grant is currently assigned to Phillips Cables Limited. Invention is credited to Shirley Beach.
United States Patent |
4,250,132 |
Beach |
February 10, 1981 |
Material and apparatus for extruding a crosslinked polyolefin
product
Abstract
A method and apparatus are provided for the extrusion of a
thermoplastic material in modified form in which the thermoplastic
material is introduced into the barrel of a screw extruder, and is
heated and forced through the barrel: the material is passed
through valve means which together with the temperature of the
material is effective to produce a melt transition in the material
at a predetermined point and to thoroughly mix the material;
subsequently, the material is forced into a low pressure zone in
the barrel where a modifying agent is introduced into the material;
the resultant mass is forced out of the low pressure zone, and is
passed through a mixing means effective to intensify the dispersion
of the modifying agent in the mass, the mass is extruded and the
extruded material collected.
Inventors: |
Beach; Shirley (Vancouver,
CA) |
Assignee: |
Phillips Cables Limited
(Brockville, CA)
|
Family
ID: |
4095540 |
Appl.
No.: |
06/044,076 |
Filed: |
May 31, 1979 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
735948 |
Oct 27, 1976 |
4181647 |
Jan 8, 1980 |
|
|
431495 |
Jan 7, 1974 |
|
|
|
|
Foreign Application Priority Data
Current U.S.
Class: |
264/68; 425/113;
425/209; 264/171.19; 264/211; 425/208 |
Current CPC
Class: |
B29C
44/3442 (20130101); B29B 7/726 (20130101); B29C
48/565 (20190201); B29B 7/826 (20130101); B29B
7/421 (20130101); B29C 48/395 (20190201); B29C
48/05 (20190201); B29C 48/06 (20190201) |
Current International
Class: |
B29C
44/34 (20060101); B29C 47/38 (20060101); B29C
47/60 (20060101); B29C 47/64 (20060101); B29F
003/10 () |
Field of
Search: |
;264/174,211,68
;425/113,208,209 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Anderson; Philip
Attorney, Agent or Firm: Murphy; Kevin P. Swabey; Alan
Mitchell; Robert E.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This is a division of application Ser. No. 735,948 filed Oct. 27,
1976 now U.S. Pat. No. 4,181,647 issued Jan. 8, 1980 which is a
continuation-in-part of U.S. Ser. No. 431,495 filed Jan. 7, 1974
now abandoned.
Claims
I claim:
1. A method of producing an electric cable conductor wire insulated
with a crosslinked polyolefin insulating material in extruded form
comprising:
introducing a solid particulate polyolefin into a barrel of a screw
extruder, containing a screw mounted for rotation, said screw being
effective to advance and shear said polyolefin,
forcing the polyolefin through the screw extruder barrel,
heating and shearing said polyolefin in an upstream portion of said
barrel, passing the polymer through combined valve and mixing means
which together with the temperature of the polymer is effective to
convert the solid, particulate polyolefin to a polymer melt, said
combined valve and mixing means comprising at least two closely
spaced apart rows of radially disposed lugs mounted normal to and
circumferentially around the screw to form a ring, the lugs of one
of said rows being in staggered relationship with the lugs of the
adjacent row,
forcing the polymer melt into a low pressure zone in the barrel and
maintaining said shearing,
introducing a crosslinking agent as a dispersion in a low viscosity
fluid into the polymer melt in the low pressure zone to form a
mixture,
forcing the resultant mass out of the low pressure zone and into a
zone of higher pressure and passing the mass through an intense
mixing means; said intense mixing means comprising at least three
spaced apart mixing elements diposed radially to said screw, each
of said elements comprising at least a pair of adjacent rows of
radially disposed lugs, the lugs of each row of a mixing element
being in a staggered relationship with the lugs of the adjacent
row, forcing the mixture through an extrusion head and about a
moving conductor wire and
crosslinking the polyolefin with said crosslinking agent,
collecting the conductor wire insulated with extruded crosslinked
polyolefin.
2. A method according to claim 1, wherein said lugs of said mixing
elements comprise pins having an outer end profiled to match the
internal contour of said barrel.
3. A method of producing an electric cable conductor wire insulated
with a crosslinked polyethylene insulating material in extruded
form comprising:
introducing a solid particulate polymer of ethylene into a barrel
of a screw extruder, containing a screw mounted for rotation, said
screw being effective to advance and shear said polymer,
forcing the polymer through the screw extruder barrel,
heating and shearing said polymer in an upstream portion of said
barrel, passing the polymer through combined valve and mixing means
which together with the temperature of the polymer is effective to
produce and convert the solid particulate polymer to a polymer
melt,
forcing the polymer melt into a low pressure zone in the barrel and
maintaining said shearing,
introducing a crosslinking agent as a dispersion in a low viscosity
fluid into the polymer melt in the low pressure zone to form a
mixture,
forcing the resultant mass out of the low pressure zone and into a
zone of higher pressure and passing the mass through an intense
mixing means,
forcing the mixture through an extrusion head and about a moving
conductor wire and allowing the polymer to crosslink,
collecting the conductor wire insulated with extruded crosslinked
polyethylene;
said combined valve and mixing means comprising at least two
closely spaced apart rows of pins mounted normal to and
circumferentially around the screw to form a ring, the pins of one
of said rows being in staggered relationship with the pins of the
adjacent row;
said intense mixing means comprising at least three spaced apart
rings, each ring comprising two rows of pins mounted normal to and
circumferentially around the screw and the pins of one of said rows
in each ring being in staggered relationship with the pins of the
other of said rows in each ring.
4. A method according to claim 3, in which the cross-linking agent
is a peroxide compound.
5. A method according to claim 4, in which the peroxide compound is
dicumyl peroxide.
6. A method according to claim 3, in which the polyethylene is low
density polyethylene.
7. A screw extrusion apparatus for working and modifying polymer
material, which apparatus comprises a barrel leading from a feed
port to a discharge; electrical resistance heaters in said barrel
for heating polymer material during passage through said barrel; a
continuous feed screw provided with a core and helical thread
rotatable in said barrel, said core having upstream and downstream
portions interconnected by an intermediate portion of lesser
diameter forming a low pressure extrusion zone; an intermediate
passage for introducing liquid or gaseous modifying substance into
said zone; intense mixing means in said downstream portion; and a
combined valve and mixing means on said upstream portion; said
combined valve and mixing means comprising at least two closely
spaced apart rows of radially disposed lugs mounted normal to and
circumferentially around the screw to form a ring, the lugs of one
of said rows being in staggered relationship with the lugs of the
other of said rows; and said intense mixing means comprising at
least three spaced apart mixing elements disposed radially to said
screw, each of said elements comprising at least a pair of adjacent
rows of radially disposed lugs, the lugs of one row of a mixing
element being in a staggered relationship with the lugs of the
other of said rows.
8. Apparatus according to claim 7, wherein based on a screw having
a L/D ratio of 24:1, said intense mixing means comprises 3 or 4 of
said spaced apart mixing elements, each of said elements comprising
2 or 3 of said adjacent rows.
9. Apparatus according to claim 8, wherein said intense mixing
means includes not more than one additional mixing element for each
additional 4 L/D.
10. Apparatus according to claim 8, wherein said lugs of said
combined valve and mixing means and said mixing elements comprise
pins and said combined valve and mixing means comprises 2 or 3 rows
of said pins.
Description
BACKGROUND OF THE INVENTION
(a) Field of the Invention
This invention relates to a process and apparatus for producing an
extruded thermoplastic polymer material in modified form.
In one particular aspect the invention relates to a process and
apparatus for the production of extruded insulating coatings in
cellular or cross-linked form for wire and cable.
(b) Description of Prior Art
In the production of wire and cable having an insulating coating, a
thermoplastic material is extruded through a screw extruder and
around a wire or cable passing through a die in the head of the
screw extrusion apparatus. In accordance with the properties
required in the coated wire or cable, the thermoplastic material is
suitably modified.
For example, if a cellular effect is desired in the insulating
coating, then a gaseous material may be incorporated in the
thermoplastic material in the screw extruder barrel. The gaseous
material may be formed in the thermoplastic material by the
introduction of a solid blowing agent which is thermally decomposed
to produce bubbles of gas, which provide the cellular effect;
alternatively a gaseous material may be introduced directly into
the thermoplastic material. In the latter case the introduction of
an inert particulate material, for example, clay or metals is also
desirable; to provide nucleation sites for the formation of the gas
bubbles; if no nucleation sites are provided for the gaseous
material there is a tendency for the gaseous material to form a
solid solution with the thermoplastic material and no cellular
effect is produced or at best the extruded material contains only
random cells and lacks uniformity.
These methods have certain disadvantages in that the non-gaseous
decomposition products produced in the thermal decomposition of the
blowing agent and the properties of the inert particulate material
can adversely effect the electrical properties of the coated
cable.
For example, a commonly used blowing agent for the production of
cellular thermoplastic material is azodicarbonamide, which, in the
commercial form, or in the presence of commonly used additives,
when thermally decomposed produces water, which if retained in the
thermoplastic material will affect the electrical properties of the
wire or cable coated with the material.
In most domestic articles the presence of water in the extruded
cellular product would be of no significance, however, in the
communication cable industry the presence of water in the extruded
cellular insulating coatings of the cable conductors represents a
serious problem.
The most commonly employed thermally decomposable blowing agent for
the production of cellular insulated coatings in the communication
cable industry is azodicarbonamide; the azodicarbonamide used by
the communication cable industry includes a hydrated silica
additive as a plating out agent; this hydrated silica produces
water on heating which reacts with isocyanic acid produced in the
thermal decomposition of the azodicarbonamide to prevent formation
of cyanuric acid and cyamelide which would otherwise build up on
the screw and die surface in the form of a white powder or pasty
substance and interfere with the satisfactory operation of the
extrusion, as well as acting as contaminants which affect the
electrical properties of the cellular insulation.
However the production of water also results in the presence of
residual water in the cellular insulating coatings produced.
Water is also evolved directly by the thermal decomposition of
other blowing agents such as p,p'-oxybis(benzene
sulfonylhydrazide).
As discussed above in most fields of technology the amount of
moisture produced in thermal decomposition of the commercially
available particulate blowing agent would be considered
insignificant even if the user was aware of it and in any event
would be too small to produce any deleterious effect. This is not
the case, however, in the sophisticated field of communication
cable technology.
In any communication cable used at the higher frequencies (10
megahertz) which are frequencies for the video or television range,
loss of signal becomes the important characteristics and even with
the best manufactured communication cables there is a signal loss
with distance (attenuation). This signal loss increases with
frequency resulting in the need to introduce repeators
(amplifiers). The attenuation is in part dependent on the
dissipation factor and the SIC (Specific Inductor Capacitance) of
the insulator in the transmission line. For example, a properly
expanded cellular insulator which has not been put through a drying
cycle, and which thus contains residual moisture, will have a
dissipation factor of as high as 0.0006; after vacuum drying to
remove the moisture, however, the dissipation factor will drop to
0.00015 and the attenuation of the line will drop accordingly.
The normal manufacture of CATV cables for the North American market
invariably involves a prolonged drying cycle to remove residual
moisture in the cellular insulation; this drying cycle is performed
in an oven of low relative humidity (about 5%) at 150.degree. C. or
in a vacuum bell at 150.degree. C., prior to application of the
outer conductor and sheath of the cable. This moisture must be
removed before applying the outer conductor, since otherwise the
outer conductor will hermetically seal the moisture in after which
it would be impossible to remove more moisture from the cable.
In conventional practice reels of the cables which may contain 15
to 20 layers on a reel are dried in ovens or vacuum bells as
described for periods of the order of 48 hours.
Clearly, therefore, if the presence of residual moisture can be
avoided or significantly reduced, the prolonged drying cycle can be
eliminated, thereby resulting in considerable economy both in time
and money.
Similarly the presence of inert particles of clay or metals as
nucleation sites in the thermoplastic material will affect the
electrical properties, for example, the capacitance of a wire or
cable coated with the material.
In one aspect the present invention seeks to overcome these
problems by providing a method and apparatus for producing an
extruded cellular thermoplastic insulating coating on a
communication cable conductor wire wherein a small amount of a heat
decomposable material is used to provide nucleation sites for a
gaseous material and the use of non-heat decomposable particulate
materials such as clay and metals as nucleation sites can be
avoided; in this case the residual non-gaseous thermal
decomposition products formed from the decomposition of the small
amount of heat decomposable material are at a minimum, and so any
adverse effect is also minimized and the drying cycle can be
reduced or minimized.
It is also conventional practice to modify thermoplastic polymer
material by the incorporation of a crosslinking agent in the
thermoplastic material, particularly in the power cable industry.
The cross-linking agent is incorporated in the thermoplastic
polymer material prior to the introduction of the material into the
screw extruder. Generally, the supplier of the thermoplastic
material incorporates the crosslinking agent in the material so
that it will set when heated to an elevated temperature. This has
inherent disadvantages in that the customer has no control over the
content of cross-linking agent in the thermoplastic material
supplied and individual batches from the supplier vary with
consequent variation in the electrical properties of the coated
wire or cable. Also, cross-linking agents are often extremely
unstable and some cross-linking inevitably occurs during storage of
the thermoplastic material containing the cross-linking agent and
this makes the production of a homogeneous mixture more difficult
during extrusion in the screw extruder.
In the conventional procedure the supplier of the polymer
incorporates the cross-linking agent into the polymer while it is
in a molten state and thoroughly mixes the molten polymer with the
cross-linking agent under carefully controlled conditions in an
attempt to minimize any cross-linking. After the mixing is complete
the molten polymer containing the cross-linking agent is cooled and
stored ready for shipment to the cable manufacturer.
This somewhat unsatisfactory procedure arises from the difficulty
in handling molten polymers with crosslinking agents without
prematurely initiating the crosslinking. The normal shearing action
which occurs in a screw extruder is not adequate to produce
thorough mixing of the cross-linking agent and attempts to improve
the mixing by increasing the temperature of the molten polymer to
lower its viscosity result in premature cross-linking which in turn
results in blockages in the screw extruder, necessitating
dismantling of the extruder for cleaning to remove cross-linked
material.
The mixing means employed on the screw in the method of the present
invention, however, overcome this difficulty and produce thorough
mixing without premature cross-linking in the screw extruder.
In one aspect of the present invention there is provided an
improved method and apparatus for producing a cross-linked
polymeric extruded insulating coating in which the user is able to
control the content of cross-linking agent directly.
It is an object of this invention to provide an improved method for
producing cellularly insulated wires, particularly conductor wires
for communication cables.
It is a further object of this invention to provide an improved
method for producing a cross-linked insulating coating on a wire,
particularly a conductor wire for a power cable.
It is a further object of this invention to provide a novel screw
extrusion apparatus having a novel valve means and mixing
means.
SUMMARY OF THE INVENTION
According to the invention there is provided a method of preparing
a cable conductor wire insulated with a cellular thermoplastic
electrically insulating material in extruded form comprising:
introducing a solid thermoplastic electrically insulating polymer
material and a small amount of a discrete particulate material
effective to provide nucleation sites into a screw extruder barrel,
containing a screw mounted for rotation, to form a mixture, said
screw being effective to advance and shear said mixture, passing
the mixture through the screw extruder barrel, heating and shearing
said mixture in a first portion of said barrel to melt the polymer
material, passing the molten mixture through a combined valve and
mixing means which together with the heating and shearing of the
mixture is effective to produce the melting of the polymer material
from the solid state to a molten state and to disperse the
nucleation sites in the molten mixture, forcing said molten mixture
into a low pressure zone in the barrel and maintaining said
shearing, introducing a gaseous material into said molten mixture
in the low pressure zone, and allowing the gaseous material to be
nucleated in the molten mixture at nucleation sites provided by
said particulate material, forcing the resultant mass out of the
low pressure zone, and into a zone of higher pressure and passing
the mass through an intense mixing means effective to disperse the
nucleated gaseous material said mixing means comprising at least
three spaced apart mixing elements disposed radially to said screw,
each of said elements comprising at least a pair of adjacent rows
of radially disposed lugs, the lugs of each row of a mixing element
being in a staggered relationship with the lugs of the adjacent
row, forcing said mixture through an extrusion head and about a
moving conductor wire in a compressed form and allowing the thus
obtained extruded mixture to expand, collecting the conductor wire
insulated with cellular thermoplastic extruded insulating material;
said valve means comprising at least two closely spaced apart rows
of radially disposed lugs mounted normal to and circumferentially
around the screw to form a ring, the lugs of one of said rows being
in staggered relationship with the lugs of the other of said
rows.
According to another aspect of the invention there is provided a
method of producing an electric cable conductor wire insulated with
a crosslinked polyolefin insulating material in extruded form
comprising: introducing a polyolefin into a barrel of a screw
extruder, containing a screw mounted for rotation, said screw being
effective to advance and shear said polyolefin, forcing the
polyolefin through the screw extruder barrel, heating and shearing
said polyolefin in a first portion of said barrel to produce a
transistion in the polyolefin from the solid state to a molten
state, forcing the polyolefin into a low pressure zone in the
barrel and maintaining said shearing, introducing a crosslinking
agent as a dispersion in a low viscosity fluid into the polyolefin
in the low pressure zone to form a mixture, forcing the resultant
mass out of the low pressure zone and into a zone of higher
pressure and passing the mass through an intense mixing means, said
mixing means comprising at least three spaced apart mixing elements
disposed radially to said screw, each of said elements comprising
at least a pair of adjacent rows of radially disposed lugs, the
lugs of each row of a mixing element being in a staggered
relationship with the lugs of the adjacent row, forcing the mixture
through an extrusion head and about a moving conductor wire and
allowing the polyolefin to crosslink, collecting the conductor wire
insulated with extruded cross-linked polyolefin.
According to a still further aspect of the invention there is
provided a screw extrusion apparatus for working and modifying
polymer material, which apparatus comprises a barrel leading from a
feed port to a discharge; means for heating polymer material during
passage through said barrel; a continuous feed screw provided with
a core and helical thread rotatable in said barrel, said core
having upstream and downstream portions interconnected by an
intermediate portion of lesser diameter forming a low pressure
extrusion zone; an intermediate passage for introducing a modifying
substance into said zone; material mixing means in said downstream
portion; and a combined valve and mixing means on said upstream
portion; said combined means comprising at least two closely spaced
apart rows of radially disposed lugs mounted normal to and
circumferentially around the screws to form a ring, the lugs of one
of said rows being in staggered relationship with the lugs of the
other of said rows; and said mixing means comprising at least three
spaced apart mixing elements disposed radially to said screw, each
of said elements comprising at least a pair of adjacent rows of
radially disposed lugs, the lugs of one row of a mixing element
being in a staggered relationship with the lugs of the other of
said rows.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(i) Cellular Extrusion
This embodiment of the invention provides for the production of
cellular insulating coatings on conductor wires, particularly those
intended as a component of a communication cable.
By employing the method of the invention cellularly insulated
conductor wires for communication cables can be passed directly
from a wire covering crosshead die at the extruder to the assembly
stage of a communication cable, without the necessity of a
prolonged drying operation to remove moisture, thereby speeding up
the total assembly line for communication cable manufacture.
Typical thermoplastic polymers which may be employed for the
manufacture of cellular coatings include polymers or mixtures of
polymers obtained by the polymerization or copolymerization of
aliphatic olefins, for example, ethylene, propylene and butene;
halogenated aliphatic olefins, for example vinyl chloride. In the
manufacture of cellularly insulated conductor wires for
communication cables, polyethylene and polypropylene are especially
preferred. Particularly useful copolymers include copolymers of
ethylene and butene and copolymers of propylene and ethylene.
Suitable gaseous materials include nitrogen, argon, helium, air,
carbon dioxide pentane, fluorinated lower hydrocarbons and lower
hydrocarbons which are both fluorinated and chlorinated, typical of
the fluorinated and chlorinated compounds are those available under
the trademark "FREON", for example, trichlorofluoromethane,
dichlorodifluoromethane, chlorotrifluoromethane,
chlorodifluoromethane 1,2,2-trichloro, 1,2,2-trifluoromethane, and
symdichlorotetrafluoroethane.
Suitable blowing agents include azodicarbonamide,
2,4'-oxybis(benzenesulphonylhydrazide), N-amino-phthalamide and
N,N'-dinitropentamethylenetetramine, with azodicarbonamide being
preferred. These blowing agents decompose to produce gas bubbles
which provide nucleation sites for the gaseous material. Further
some of the blowing agents produce water on thermal decomposition.
In the manufacture of cellular insulation coatings for
communication cable conductor wires such blowing agents contain an
anti-plating agent which is usually a hydrated silica which on
heating evolves water. Thus water is evolved from the blowing agent
additive during the heating in the extruder even though the water
is not necessarily produced by thermal decomposition of the blowing
agent. The production of water in this way is intentional to
hydrolyse non-gaseous by-products of the thermal decomposition of
the blowing agent.
The method of the invention while employing the conventional
blowing agent additive minimizes the amount employed thereby
minimizing residual moisture in the cellular insulation to an
acceptable level.
The silica produced from the hydrated silica assists in the
formation of nucleation sites, by providing points of high heat
content for gas nucleation; however, its presence for this purpose
is not essential to this invention.
Suitably the blowing agent additive is employed in an amount of 0.1
to 0.2% and preferably about 0.15%, by weight, based on the weight
of thermoplastic material.
(ii) Cross-linked Extrusion
This embodiment of the invention provides for the production of
cross-linked insulating coatings on conductor wires, particularly
those intended as a component of a power cable.
The method of the invention permits the manufacturer of the power
cable to introduce the cross-linking agent directly into the molten
cross-linkable polymer material in the screw extruder and to obtain
thorough mixing of the cross-linking agent with the polymer
immediately prior to the cross-linking step.
This enables the cable manufacturer to control the amount of
cross-linking agent, and reduces to a minimum the time which the
cross-linking agent is in the polymer prior to the cross-linking
step, thereby avoiding the possibility of premature
cross-linking.
Suitable polymers include any of the conventional polymers which
can be cross-linked; polyolefins, for example, polyethylene and
polypropylene are particularly preferred, with low density
polyethylene being especially preferred.
Suitable cross-linking agents are the peroxides, for example,
dicumyl peroxide, dibenzoyl peroxide, t-butyl perbenzoate
2,5-bis(ter-butylperoxy)-2,5-dimethylhexane and
2,5-dimethyl-2,5-di(t-butylperoxy).
The cross-linking agents are conveniently introduced as a
dispersion in a low viscosity fluid, typically in the form of a
lower viscosity paste.
A suitable paste may be produced by intensely mixing a peroxide
cross-linking agent, for example, dicumyl peroxide with a low
molecular weight polybutene in an ink mill or similar three roll
mill, using a heated premixed mixture of the polybutene and the
dicumyl peroxide. The paste may be introduced continuously into the
extruder barrel by a balloon pump although other means of
introduction will be readily apparent.
The amount of cross-linking agent added is suitably of the order of
about 1% to about 3% preferably about 2%, by weight, based on the
weight of polymer.
(iii) Screw Extruder
The screw extruder of the invention includes both a valve means in
an upstream portion of the screw and a mixing means in a downstream
portion.
It has been found that in a screw extruder the conversion of a
solid polymer to a polymer melt must occur or at least be initiated
within that upstream region of the barrel containing the first five
turns of the helical flight from the feed end. Generally if the
conversion of the solid polymer to the polymer melt does not occur
in this region it becomes impossible to advance the polymer through
the screw extruder.
This factor is taken account of in determining the form and
location of the valve means and mixing means, but especially the
latter.
Considering the mixing means this comprises at least three spaced
apart mixing elements disposed radially to the screw, each of which
elements comprises at least a pair of adjacent rows of radially
disposed lugs, the lugs of each row of a mixing element being in a
staggered relationship with the lugs of the adjacent row so as to
provide a tortuous path for the polymer serving to divide the
molten material into streams which re-unite after passing each
element, thereby producing an intensive mixing.
The lugs extend radially outwardly from the screw so that only a
narrow clearance is provided between the ends of the lugs and the
inner surface of the barrel.
Each row of lugs is suitably a row of pins the ends of which have a
profile which is convexly curved to match the concave curvature of
the inner surface of the barrel. Pins of this form have been found
to be superior to pins having squared ends. Each row may similarly
be composed of a toothed ring, but again toothed rings in which the
teeth have rectangular ends are found to be less satisfactory than
pins with rounded ends. It is believed that the narrow clearance
between the pins with rounded ends and the inner surface of the
barrel assists in forcing the polymer to take the tortuous path
provided between the staggered pins.
A conventional screw has a length to diameter ratio (L/D ratio) of
24:1. For this screw it is found that the mixing means should
desirably consist of only 3 or 4 spaced apart mixing elements each
of which comprises 2 or 3 spaced apart rows of the staggered pins.
If there are four spaced apart elements then there are preferably
only two spaced apart rows in each element.
Optimum results are obtained with three mixing elements each
comprising two rows of pins in staggered relationship. If less than
three mixing elements are employed then the mixing obtained is
inadequate, if more than four mixing elements are employed in this
screw then undesirably high temperatures are generated by the very
intense mixing. Further as the number of mixing elements is
increased the point at which the solid polymer is converted by
melting to a polymer melt, moves down stream in the extruder, with
the result that it may not take place within the region of the
first five turns of the helical flight.
However, if the screw extruder is lengthened then additional mixing
elements may be added although they may be unnecessary to obtain
adequate mixing. Generally it is found that one additional mixing
element comprising a double or a triple row of pins can be added
for each additional 4 L/D of screw extruder. In other words if the
L/D ratio of the screw is increased from 24:1 to 28:1, then one
additional mixing element can be accommodated. If the L/D is
increased to 32:1 then two additional mixing elements can be
accommodated.
The valve means comprise at least two closely spaced apart rows of
lugs mounted normal to and circumferentially around the screw to
form a ring, the lugs of one of the rows being in a staggered
relationship with the lugs of the adjacent row. The closely spaced
rows and the staggered relationship of the lugs provide a tortuous
path for the polymer. The valve means produces a back pressure
which produces an additional mixing effect supplementing the mixing
action provided by the lugs which divide the polymer into narrow
streams which re-unite.
The lugs of the valve means are preferably pins of the same form as
the pins of the preferred mixing elements. Preferably the valve
means comprises only 2 or 3 rows of the pins.
In the embodiment of the invention relating to cellular extrusion
the valve means provides an intense mixing of the polymer melt and
nucleation sites prior to the introduction of the polymer into the
low pressure zone, where the gaseous material to be nucleated is
introduced. This serves to distribute the nucleation sites through
the polymer melt.
The valve means has a further function in that it effectively seals
off the upstream portion of the screw extruder to escape of gaseous
material introduced into the low pressure zone, where the gaseous
material might either escape from the feed end of the extruder or
enter into a solid solution. The valve means forms an obstacle to
the passage of the polymer melt through the screw extruder and
produces a back pressure which itself produces a mixing effect and
a high pressure zone which effectively prevents the escape of the
gaseous material.
A further feature of the valve means is that it is a determining
factor in the position in the screw extruder where the polymer
melts it is not the only factor, however, and the temperature of
the material and the mixing means in the downstream portion of the
screw are also factors. Nevertheless the preselection of the kind
and location of the valve means is decisive in predetermining the
position of formation of the polymer melt. This is highly
significant because, as has already been described, it appears to
be critical to produce the polymer melt close to the feed end of
the screw for successful continuous operation.
The valve means is not essential in the embodiment of the invention
relating to cross-linking, nevertheless it is a highly desirable
feature of this aspect, in view of its ability to determine the
position of the polymer melt formation. Further when the polymer
contains additives, for example, pigments, the intense mixing
produced by the valve means assists in thoroughly dispersing
these.
The screw of the screw extruder has a helical flight which is
substantially continuous from the feed end to the die end. The
flight is however interrupted at the locations where the pins of
the valve means and mixing means are located; these locations where
the flight is interrupted are thus very short in length being just
sufficient to permit mounting of the pins so that a complete ring
of pins is formed. Thus there are no flightless portions of the
screw.
BRIEF DESCRIPTION OF DRAWINGS
A preferred embodiment of the invention is illustrated with
reference to the accompanying drawings, in which:
FIG. 1 is a cross-sectional view of an extruder,
FIG. 2 is a cross-sectional view of a wire covering crosshead die
used in extruding coatings about a wire conductor.
FIG. 3 is a cross-section on a line 3--3 of FIG. 1 and
FIG. 4 illustrates a toothed ring suitable as the valve means.
DESCRIPTION OF THE PREFERRED EMBODIMENTS WITH REFERENCE TO THE
DRAWINGS
With reference to FIG. 1 and extruder 1 has a multi-zone two-stage
feed screw 2 rotatably mounted in a barrel 3 heated by means of
electrical resistance heaters 4.
The barrel 3 of the extruder is fitted with a feed hopper 5 having
a feed port 6 situated at the base thereof, an inlet passage 7
having a nozzle 25 for the introduction of a modifying agent into
the barrel 3, thermocouples 8, pressure gauges 9 and a die 10.
The feed screw 2 comprises a downstream portion 11 and an upstream
portion 12 intervened by an intermediate portion 13 of lesser
diameter and by tapered portions 14 and 15.
The zone in the barrel 3 occupied by the intermediate portion 13
constitutes, due to the smaller diameter of intermediate portion
13, a low pressure zone in the barrel 3 relative to the zones on
each side of the intermediate portion 13.
A valve 16 comprising two rows of pins and a mixing element 17
comprising mixing rings 17a, 17b, 17c each of which comprises two
rows of pins the rows of pins in valve 16 and mixing element 17 are
located circumferentially around the screw 2, the pins of one row
of a pair being staggered relative to the pins of the other row of
the pair, and are mounted in spaced apart relationship on the feed
screw 2, the value 16 being mounted on the portion 12 and the rings
17a, 17b and 17c being mounted on the portion 11.
In the operation of the process for modifying a thermoplastic
material, a thermoplastic polymer is fed into the extruder 1 via
the hopper 5 and feed port 6. The screw 2 driven by suitable drive
means (not shown) advances the thermoplastic polymer through barrel
3, which is heated by heaters 4. The thermoplastic polymer is
thereby forced into intimate and substantially sliding contact with
the hot walls of barrel 3, and is also sheared and worked whereby
frictional effects are produced.
The thermoplastic material is forced through barrel 3, and a
conversion or melt transition is produced in it, from a solid
particulate state to a fluid or molten state i.e.; a polymer melt,
by the combined effect of the valve 16, the temperature of the
material and to a lesser extent the mixing element 17. It has been
found that the position of the melt transition can be predetermined
to a large extent by preselection of the form and location of valve
16 and of the temperature maintained in the barrel 3; and that the
position is not affected by the dimensions of the die 10, as is
believed to be the case when no valve 16 is used.
The ability to select the point of polymer melt formation clearly
has a number of advantages. For example, where modifying substances
are to be added to the material, the state of the material can be
appropriately selected at a given point relative to the
introduction of the modifying material for a more efficient
operation.
The material is subjected to shearing forces to produce mixing and
a back pressure produced by the valve 16. This back pressure
produces an additional mixing effect, which is supplemented by the
mixing action of the pins of the valve 16, which serve to divide
the molten thermoplastic material into streams, which reunite after
passing the valve 16. The valve 16 produce a thorough mixing of the
thermoplastic material before it proceeds into the low pressure
zone around intermediate portion 13, where the modifying agent is
introduced into the thermoplastic material via nozzle 25 of inlet
passage 7.
The thermoplastic material and modifying agent proceed out of the
low pressure zone, and are subjected to a mixing action by the
mixing rings 17a, 17b and 17c, which serve to intensify the
dispersion of the modifying agent in the thermoplastic
material.
In the case in which the modifying agent is a gaseous material and
a cellular or foamed structure is to be formed a small amount, for
example 0.1% of a discrete particulate preferably heat decomposable
gas producing material as for example azodicarbonamide is
introduced into the thermoplastic polymer via hopper 5. This
material decomposes in the barrel 3 around the upstream portion 12
to produce small gas bubbles, which will act as nucleation sites
for the gaseous material introduced in the low pressure zone. Other
materials in particulate form, for example, clay and metal can also
be used to provide nucleation sites, however, these are less
preferred since they remain in the extruded material and
consequently may affect the electrical properties. Since only a
small amount of a heat decomposable material is used, residual
amounts of non-gaseous thermal decomposition products are at a
minimum.
The valve 16 provides a thorough mixing and consequently a uniform
distribution of the nucleation centres in the thermoplastic
material as well as aiding the melt transition as described above.
The mixing rings 17a, 17b and 17c intensify the dispersion of the
gaseous material introduced via inlet 7 to maintain the uniform
distribution of gas bubbles in the thermoplastic material.
The valve 16 is also effective in preventing loss of the gaseous
material introduced via inlet 7 through the feed end of the barrel
3.
The amount of gaseous material introduced into a particular polymer
for a specific wall thickness and for a specific end use of the
product will in practice vary by volume from about 40% of
dielectric volume to 70% of dielectric volume for olefin polymers
and as much as 90% for styrene polymers. For example, in an
insulated conductor in which the starting olefin polymer has a
density of 0.93 (units) the density is reduced by the addition of
gaseous material, for example, nitrogen, to a figure which will
indicate the above-mentioned percentages of gas.
Since in many instances there is a desired capacitance effect as
well as a desired thickness of insulation for a particular wire or
conductor, the amount of gas introduced is that which will produce
the desired parameters and consequently may be less than the total
amount of gas which it is possible to introduce.
Gas is introduced continuously through passage 7 at a rate that
will satisfy the parameters of the desired end product and which
will allow a pressure of introduction sufficient to overcome the
pressure in the barrel 3 attempting to obstruct the gas. Specific
nozzle sizes for the passage 7 and specific pressures are required
for a wide variety of end products, however, these parameters are
readily determinable, generally it has been found that an air
driven pump having a compression ratio of 20:1 is suitable.
Also, it would appear that the introduction of the gaseous material
through a single nozzle 25 in passage 7 of small size is
significant and, particularly for the insulation of telephone
cables, a single nozzle 25 having a hole diameter of about 0.002
inches has produced satisfactory results where the pressure in the
low pressure zone around intermediate portion 13 of screw 2 is of
the order of 2000 to 4000 psi. In this case, it is necessary to use
a pump to introduce the gaseous material with a uniform output to
overcome the pressure. Therefore, the amount of gaseous material
passing through the nozzle 25 is a function of both the pump
pressure and the hole size.
A significant advantage of the process of the present invention
using the extruder 1 is that it has been found possible to achieve
good control of the capacitance of the insulated wire to a
permissible deviation of plus or minus 0.5 picofarads/foot.
In the case in which a cellular insulation is produced by the
introduction of a gaseous material the coated material is quenched
in a water bath to limit expansion of the gas; the water bath is
moved automatically to change the position of the water quenching
interface upon instruction or signal from a capacitance monitor
through an amplifier or servo and the final capacitance is
monitored in this way. However, the final capacitance is dependent
on several factors including the composition and uniformity of the
insulating material. The present invention in providing a coating
material of uniform composition and consistency thus greatly
assists in the control of capacitance of the insulated wire.
The material is extruded through the die 10, and the gas bubbles
expand to produce the cellular structure.
In the case in which the modifying agent is a cross-linking agent,
the rings 17a, 17b and 17c ensure a thorough distribution of the
cross-linking agent in the thermoplastic polymer; this is desirable
in order to ensure a product of uniform composition and properties.
Cross-linking not only produces a heat settable product, which can
thus be used at high temperatures at which the non-cross-linked
polymer would melt, but also for certain polymers, for example
polyethylene, the tensile strength and resistance to stress
cracking are increased.
The cross-linking agent is suitably added as a low viscosity paste
comprising the cross-linking agent in a liquid vehicle. In one
embodiment a suitable paste comprising dicumyl peroxide in a low
molecular weight polybutene was made in an ink mill using a heated
pre-mixed solution of low molecular weight polybutene and dicumyl
peroxide. The paste may be introduced continuously into the
extruder barrel by a balloon pump.
The process and apparatus according to the invention are
particularly suitable for the extrusion of a coating around a
conducting wire in the manufacture of electrical cables,
particularly communication cables and power cables.
FIG. 2 is a cross-sectional view of a typical wire covering
crosshead employed in the manufacture of electrical cables. With
reference to FIG. 2, a crosshead 24 comprises a breaker plate
assembly 18, a conducting wire 19, a guide passage 20 for the wire
19, a guide mandrel 21, and an orifice 22.
With further reference to FIG. 3, a plurality of pins 30 and 31 are
mounted in the screw 2, the exposed ends of the pins being convexly
curved, the pins 30 and 31 forming two rows respectively the pins
30 being in staggered relationship with the pins 31 so that each
pin 30 is located exactly opposite the space between two pins 31
and vice versa.
The pins of rings 17a, 17b and 17c are of similar form to those of
pins 30 and 31 of valve 16.
With reference to FIG. 4, a detail of a mixing ring 16a of a valve
16 having teeth 23 rather than a plurality of pins is shown.
The valve 16 is located in the screw 2 such that the pins 30 in one
ring of the valve 16 fall exactly between the spaces between the
pins 31 in the other ring of the valve 16 (see FIG. 3); and the
rings of the valve 16 are spaced such that material passing between
the rings of the valve 16 has to travel along a tortuous path for
example 1/16 inches wide between each pin 30 in the first row of
pins and corresponding pin 31 in the second row of pins.
The clearance between the upper end of pins 30 and 31 and the wall
of the barrel 3 is about 0.008 inches to about 0.06 inches and may
be the same as between the screw flights and the barrel.
In the embodiment illustrated in FIG. 1 the valve 16 and mixing
rings 17a, 17b and 17c each comprise a pair of rows of pins spaced
circumferentially around the screw 2.
The locations and spacings of valve 16 and rings 17a, 17b and 17c
relative to the tip of the screw 2 at the downstream extension end
in a particular embodiment, for a screw 2 of the dimension given in
Table I below, are given in Table II below. In this specific
embodiment detailed in Table I, upstream portion 12 comprises three
zones namely the first, second and third zones of the screw. The
first zone is the feed section; the second zone is a tapered
transition section; the third zone is a first metering section. The
dimensions given are by way of illustration only, and it will be
readily apparent that other dimensions could be used which could be
readily determined by experiment.
TABLE I ______________________________________ inches
______________________________________ length of screw overall
76.375 length of downstream portion 11 20 length of intermediate
portion 13 2.5 length of upstream portion 12 39.25 first zone of
portion 12 7 second zone of portion 12 22.25 third zone of portion
12 10 axial length of tapered portion 15 2.5 axial length of
tapered portion 14 1.25 outside diameter of screw 2 2.5 constant
depth of thread at downstream portion 11 0.15 nominal depth of
thread at intermediate portion 13 0.300 nominal depth of thread at
upstream portion 12 first zone of portion 12 0.330 nominal second
zone of portion 12 tapered third zone of portion 12 0.110 nominal
______________________________________
The overall diameter of the screw 2 is constant throughout (from
thread tip to tip), and the diameter of the core of the screw 2
varies according to the depth of the thread.
In the exemplified embodiment tapered portion 14 constitutes a
fourth zone of the screw; intermediate portion 13 constitutes a
fifth zone being the vent section; tapered portion 15 constitutes a
sixth zone; and downstream portion 11 is the seventh zone being a
second metering section.
TABLE II ______________________________________ Rings d.sub.1
inches d.sub.2 inches d.sub.3 inches
______________________________________ 16 0.118-.005 0.030 26.5 17a
0.2162-.003 0.120 17.5 17b 0.1447-.005 0.060 11.25 17c 0.118-.005
0.030 5 ______________________________________ d.sub.1 is the
distance of separation between the adjacent rows of pins, d.sub.2
is the distance between a pin in one row and the closest pin in the
other row, d.sub.3 is the distance of the rings from the die end of
the screw 2 at the downstream end.
In operation the material in the barrel 3 is forced through the
breaker plate 18 and passes to the guide mandrel 21, which is
shaped so that the material flows around either side of it, thereby
forming a flowing annulus around the mandrel 21, which flows
towards the orifice 22 and ultimately contacts the wire. In this
manner, a coating is deposited over the wire 19, which moves
continuously through the crosshead and acts as an internal forming
mandrel. In order to ameliorate the adhesion of the coating to the
wire 19 it is found in certain cases to be advantageous to heat the
wire 19 which heating can be achieved by passing an electric
current through the wire; generally it is desirable to heat wire 19
to a surface temperature of about 200.degree. F. and to maintain
this temperature with respect to wire throughput.
Thus is producing a coated wire for an electrical cable the
extruded polymer emerges from the die 10 and passes around the
mandrel 21 and onto a conducting wire 19 which passes continuously
through passage 20. The coated wire emerges through orifice 22.
In the embodiment in which a cellular coating is being formed the
high compressed gas bubbles in the thermoplastic polymer expand as
the coated wire emerges from orifice 22, to produce the cellular
effect. The degree of expansion is carefully controlled by cooling,
as with a moving water trough; the expansion can thus be controlled
to provide substantially uniform capacitance along the coated
conductor wire.
In the embodiment in which a cross-linked coating is being formed a
tube containing high pressure steam is mounted on the crosshead 24
adjacent orifice 22 to effect the cross-linking with the
cross-linking agent. The steam may suitably be at a temperature of
440.degree. F. to 460.degree. F.
It will be apparent to those skilled in the art that other
modifying agents might be introduced in accordance with the
invention depending on the modification of the thermoplastic
required. Additives, for example fillers, antioxidants and pigments
can also be added in accordance with the invention to modify the
properties of the thermoplastic material particularly if a
cross-linking agent is also to be added.
The following examples are illustrative of the processor according
to the invention.
EXAMPLE 1
99.85 grams of polyethylene granules (low density), and 0.15 grams
of azodicarbonamide were charged through hopper 5 into the barrel 3
of the extruder 1 having been preheated to a temperature ranging
from about 18.degree. to 55.degree. C. in the vicinity of the
hopper 5 to about 155.degree. C. in the vicinity of inlet 7 to
about 288.degree. C. in the vicinity of the die 10. Nitrogen gas
was introduced into the low pressure area under a pressure of about
2000 psi, and the mixture was extruded around a wire 19 preheated
to a surface temperature of about 93.degree. C. of approximately
0.020 inches in diameter to produce a cellular coating 0.025 inches
in thickness.
EXAMPLE 2
The process according to Example 1 was repeated with the exception
that the nitrogen gas was introduced into the low pressure area
under a pressure of about 200 psi, and the mixture was extruded
around a CATV wire (a trunk cable used for the distribution of
wired television) 19 having a diameter of about 0.5 inches.
EXAMPLE 3
98.5 grams of polyethylene granules (low density) were charged
through hopper 5, the barrel 3 of the extruder having been
preheated to a temperature ranging from about 94.degree. C. in the
vicinity of hopper 5 to about 127.degree. C. in the vicinity of
inlet 7 to about 131.degree. C. in the vicinity of die 10. 1.5
grams of dicumyl peroxide in the form of a paste in a low molecular
weight polybutene were introduced into the low pressure zone via
inlet 7 and the resultant mixture was extruded around a power cable
19 having a diameter of 0.0665 inches. The coated wire 19 emerges
from orifice 22 and passes through a 200 foot high pressure tube
containing saturated steam at a pressure of about 200 lbs/sq.in. in
which the cross-linking takes place. The resulting wire had a
coating of 0.032 inches in thickness.
* * * * *